1. Field of the Invention
The present invention is related to the field of electric wireline tools used to withdraw samples of fluids from earth formations. More specifically, the present invention is related to methods of determining various properties of fluids in earth formations by interpreting pressure and temperature readings made by electric wireline formation testing tools.
2. Description of the Related Art
Electric wireline formation testing tools are used to withdraw samples and to make pressure and temperature measurements of fluids contained within pore spaces of earth formations. Calculations made from these measurements can be used to assist estimation of the total fluid content within the earth formations.
As known in the art, a formation testing tool is typically lowered at one end of an armored electrical cable into a wellbore drilled through the earth formations. The formation testing tool typically includes a housing from which a tubular probe is extended and impressed onto the wall of the wellbore. The probe typically is externally sealed by an elastomeric packing element to exclude fluids from within the wellbore itself from entering the interior of the probe while fluids are withdrawn from the earth formation through the probe. Various selective valves in the tool place the probe in hydraulic communication with sample chambers included in the tool. Hydraulic lines which connect the probe to the various sample chambers can also be connected to a highly accurate pressure sensor for measuring the fluid pressure within the hydraulic lines. Other sensors in the tool can make measurements related to the volume of fluid which has entered some of the sample chambers during a test of a particular earth formation. The formation testing tool can also include a sample tank which can be selectively hydraulically connected to the probe so that a quantity of fluid withdrawn from the formation can be dispensed into the sample tank and transported to the earth's surface for laboratory analysis.
It is important to the wellbore operator to be able to determine that the fluid actually dispensed into the sample tank consists primarily of native fluid from within the pore space of the earth formation. In certain circumstances fluids other than the native fluid can be dispensed into the sample tank. For example, when a wellbore is drilled through the earth formations, it is typically filled with a liquid suspension, called "drilling mud". Drilling mud usually has a specific gravity great enough to exert hydrostatic pressure against the earth formations which can restrain the native fluids in the formations from entering the wellbore. It is even more typical for the hydrostatic pressure of the drilling mud to at least slightly exceed the native fluid pressures in the formations. If the drilling mud has a higher hydrostatic pressure than the native fluid pressure, the liquid phase of the drilling mud, called "mud filtrate", can be forced into the pore space in the formation by differential pressure in a process known as "invasion". An indeterminate volume of mud filtrate will be withdrawn from the formation when the probe is first hydraulically connected to the sample chamber or sample tank. Further, the probe is typically substantially filled with drilling mud when it is first hydraulically connected to the sample chamber.
If sufficient mud filtrate volume invades the earth formation, it may be necessary for the test tool operator to withdraw a very large volume of fluid from the earth formation before native fluid can be extracted therefrom and dispensed into the sample tank. It is of interest to the test tool operator to be able to determine when the fluid being withdrawn from the formation comprises native fluid so that the amount of time during which the formation test tool is locked in place can be kept to a minimum for reasons of safety as is understood by those skilled in the art.
It is known in the art to determine whether the fluid being withdrawn from the earth formation comprises native fluid by making certain measurements of the fluid as it is withdrawn through the probe. For example, U.S. Pat. No. 4,994,671 issued to Safinya et al, discloses the use of a near infrared spectrograph to determine the composition of fluid being withdrawn through the probe.
A drawback to the system disclosed in the Safinya et al '671 patent for determining the composition of fluid being withdrawn through the probe is that the spectrograph is sensitive to changes in transparency of a sample chamber window through which light must pass in order to make the spectrographic measurements of the fluid under test. Deposition of opaque materials on the window can reduce its transparency so as to make the spectrograph inoperative. Opaque materials can include certain high molecular weight hydrocarbons which may be present in some native fluids, or solid materials such as sand grains or clay particles which may break away and flow from the earth formation under certain conditions.
A further drawback to the system disclosed in the Safinya et al '671 patent is that gas bubbles, which may be present in some fluid samples, can disrupt the operation of the spectrograph, making it difficult to determine whether native fluid is being withdrawn into the probe. Gas can be present in some samples as a result of exsolution of dissolved gas as the sample pressure is reduced to enable flow into the sample chamber. Dissolved gas can be present in native crude oil in the earth formation.
It is an object of the present invention to provide a method of determining the properties of fluid withdrawn through the probe of a formation testing tool which does not require the use of a spectrographic sensor.
It is a further object of the present invention to provide a method of determining properties of fluid withdrawn through the probe of a formation testing tool which is functional in the presence of gas in a sample of the fluid.